Periodontitis, an inflammatory disorder caused by persistent bacterial infection, is one of the most common human infections. Actinobacillus actinomycetemcomitans (Aa) is the primary pathogen responsible for the progression of localized aggressive periodontitis (LAP). The hallmark of the pathology of LAP is severe destruction of alveolar bone prompted by inflammatory cytokines. Production of receptor activator of NFKB ligand (RANKL), an osteoclastogenic cytokine, by osteoblasts is crucial for formation of mature osteoclasts. Aa is known to be involved in the up-regulation of RANKL expression, however the molecular bases for such induction is not known. The following observations: 1) cytolethal distending toxin (CDT), a putative virulence factor of Aa is required for up-regulating RANKL expression in periodontal ligament cells, 2) many pathogen-derived molecules utilize the TLR pathway to regulate inflammatory cytokine production, 3) TLRs are found expressed by osteoblasts, and 4) direct interaction between Aa and alveolar bone has been documented in LAP patients, have led us to hypothesize that CDT of Aa induces RANKL expression by osteoblasts via TLRs. The hypothesis will be tested by pursuing two specific aims. In the first specific aim we will characterize Aa-mediated induction of osteoclastogenic signaling in vitro, and explore the functional contribution of TLR4 and MyD88 to the expression of RANKL and to in vitro osteoclastogenic activity by osteoblasts in response to Aa and its virulence factor CDT. In addition, we will determine the TLR-mediated intracellular signaling relevant to RANKL expression. In the second specific aim, we will recapitulate the functional importance of TLR4 and MyD88 in osteoclastogenesis highlighted by bone loss in vivo using wild type and TLR signaling molecule knockout mice.